1. Field of the Invention
The present invention relates generally to cellular communication networks, and more particularly, to a method and apparatus for simultaneous radio and mobile frequency transitions in wireless communications systems.
2. Discussion of the Related Art
Currently, cellular radio frequency (RF) plans generally assign radio channels, or frequencies, to cells within an RF network based on forecasts and ongoing studies of traffic patterns. Unfortunately, this scheme, referred to as Fixed Channel Allocation (FCA) does not take into account the dynamic nature of traffic in certain portions of a coverage area. For example, in a high-density business corridor, the channel requirements in a subcell A during a given time period may be higher than the capacity of subcell A, resulting in a condition referred to as call blockage. At the same time, a neighboring subcell B may have idle channels due to less-than-capacity call rates.
Another problem with FCA is that cell sites are shrinking is size as demand for capacity increases. Due to both irregularities in propagation and traffic distribution in these small cells, pre-assignment of channels becomes difficult.
With prior known techniques, implementation of a centralized intelligence to monitor channel usage in various cells and assigning channels based on need has not been practical, due to the large amount of overhead processing and messaging necessary to keep both the centralized intelligence and the individual cells apprized of channel usage.
A scheme, known in the art as adaptive channel allocation (ACA), deals with dynamic channel assignment (DCA). ACA includes the ability to dynamically allocate the entire spectrum based on need, without any frequency scanning per cell.
Other relevant channel assignment schemes include hybrid channel assignment (HCA) and borrowing channel assignment (BCA). With HCA, a combination of FCA and DCA, a portion of the total frequency channels uses FCA and the remainder use DCA. With BCA, when all the fixed channels of a cell (xe2x80x9cacceptor cellxe2x80x9d) are occupied, then the acceptor cell borrows free channels from a neighboring cell (xe2x80x9cdonor cellxe2x80x9d). In general, BCAs use some form of central control to lock out other cells from using the channel(s) borrowed by the acceptor cell. While BCA is effective under light to moderate traffic conditions, especially when compared to FCA, under heavy traffic, channel borrowing may be high enough to cause channel usage efficiency to drop drastically and increase blocking probability due to channel locking.
Another technique known in the art is referred to as segregation, a self-organizing dynamic channel allocation scheme. Channels are assigned probabilities of being clear based upon successful use of a given channel for a call. When a call arrives at a cell, the determination of which channel to use to service the call is based on the current probabilities of the channels. Carrier sense (using received signal strength measurement) is performed on the selected channel (i.e., the channel with the highest probability) to ensure that it is clear to use. If not, the probability of that channel is decreased and the next channel is tried. If that channel is clear, then the call is assigned to that channel and its probability is increased.
While segregation offers certain advantages over previously-described methods, it too suffers from uncertain deficiencies, including the fact that the time for convergence to optimal allocation is very long. In addition, the determination of carrier sense only at channel request is known to cause extensive delay. If delay is limited to a finite wait time, then performance of the system degrades rapidly.
In one aspect of the method of segregation, as in FCA, each cell is assigned its nominal channels, if any, from the available frequency spectrum, with a fixed radio assigned to each of the frequencies, respectively. Additionally, each cell may be equipped with one or more radios designated as xe2x80x9cDFA radios.xe2x80x9d In operation, idle DFA radios scan channels that may be borrowed in order to build a probability matrix. This enables the cell to determine which channels to use for traffic prior to actual channel request by a mobile. This reduces the search delay experienced in classic segregation schemes.
The algorithm for implementing the DFA technique operates as follows. During low traffic periods, FCA channels and DFA radios are assigned. Next, a scan list is determined to identify the channels that can potentially be used by a DFA radio in a subcell during its traffic peaks. The identified channels are placed in a scan list. Next, each individual cell uses its DFA radio(s) and Mobile Assisted Channel Allocation (IS-136 MACA) to cyclically scan each channel that may be borrowed to obtain reverse and forward RF information to determine whether the channel is clear. This functionality, referred to as xe2x80x9cScanning Modexe2x80x9d, is performed while fixed radios, and possibly other DFA radios, serve ordered traffic.
Probabilities of being clear are then assigned to channels, with weight given to consecutive clear scans. A scan is determined to be clear if the RF signal strength of the channel does not exceed a predefined threshold, which is determined by the network in question. The channels are arranged in an updated, ordered xe2x80x9cclear list.xe2x80x9d
Before an overload condition occurs in which an acceptor cell exhausts all its available channels to service calls, that cell will attempt to borrow channels designated in the ordered clear list, using the DFA radios that were used to construct the ordered clear list. Only those channels whose probability is higher than a predefined minimum threshold may be selected for use. A DFA radio of the acceptor cell is then associated with, or tuned to, the selected channel, at which point the DFA radio is said to be in xe2x80x9cSelection Mode.xe2x80x9d Received signal strength on the channel is continuously monitored to ensure that it is below a predefined threshold. Because of the behavior of the DFA radio during the low traffic periods, the delay in performing this step is substantially decreased, expediting call service at the highest traffic periods.
New calls or handoffs may now be serviced by both fixed radios, as well as DFA radios that are in Selection Mode. If interference is detected on the borrowed channel, the DFA radio is returned to the Scanning Mode. However, if the DFA radio is still needed to service calls, it determines another channel that is free of interference and returns to the Selection Mode. The donor cell (i.e., the cell from which a channel is borrowed) detects the usage of one of its nominal channels by an acceptor cell if close enough, and uses that channel only as a last resort. This eliminates the central control problem of channel locking by allowing that channel to be used if necessary in the donor cell. Lastly, if the cell exits the overload condition, the DFA radio is returned to the Scanning Mode, whereupon the probabilities of being clear for all the frequencies in the scan list are updated.
In another aspect of mobile telecommunication systems, current automatic frequency planning (AFP) transition solutions require calls to be removed from the radio that is changing channels. This requires a complex procedure of gradually changing frequencies without dropping calls and while trying to minimize interference problems due to the transitional frequency plan. The only other solution has been to drop all calls and transition the radios all at once. Other shortcomings are mentioned below.
When implementing a new frequency plan for a telecommunications system, many radios may need to change channels. It is possible for all calls within a given cell to be affected by the channel change. For instance, an interference problem may be created as a result of such transition schemes.
Typical intra-cell handoff is a process of common knowledge in the field of mobile radio telecommunications. Briefly, the normal intra-cell handoff process involves first identifying a new radio/timeslot to accept the call. The new radio is then set to transit SAT or DVCC and the mobile is instructed to change channel to the new radio""s channel. The mobile changes to the new channel and the call continues on the new radio and channel. During the known intra-cell handoff process, the central office or mobile switching center (MSC) switches the call trunk connections from the old radio to the new radio.
A common occurrence in adaptive channel allocation(ACA) is for a noisy call to be handed off of a noisy DFA channel onto a clear one. This requires an available radio or time slot to accept the handoff. Accordingly, additional time slots are needed.
Another problem with prior types of handoff is that the handoffs are visible to the central system control. As a result, trunking connections as controlled by the central system control may need to be changed.
It would thus be desirable to provide an improved mobile telecommunications system for facilitating handoffs including simultaneous radio and mobile frequency transitions to overcome the problems in the art.
A cellular network having an radio frequency (RF) management capability includes a mobile switching center, at least one base station controller operative coupled to the mobile switching center, and at least one base station operatively coupled to the at least one base station controller. The at least one base station is for being operatively coupled with at least one mobile station for carrying out at least one it) mobile communications call. Lastly, a radio frequency manager is operatively coupled to the at least one base station, in conjunction with the at least one mobile station. The RF manager is further for managing a handoff upon the occurrence of a prescribed event, the handoff including a handoff-to-self via a simultaneous base station radio and mobile station frequency transition.
A technical advantage achieved with the embodiments of the present disclosure is that, in an improved mobile telecommunications system, handoff-to-self facilitates simultaneous radio and mobile frequency transitions for overcoming problems in the art.